ABS (acrylonitrile-butadiene-styrene) resins generally have a good balance of properties such as impact resistance, chemical resistance, heat resistance, and mechanical strength as well as processability, and are easily molded. Accordingly ABS resins have been widely used for various products such as automobile parts, housings for electrical/electronic appliances, and the like, in addition to small household and personal items and amenities.
Conventional resin products have a solid and glossy surface. Recently, however, there has been an increasing demand for thermoplastic resins having both low gloss and a soft touch surface for interior parts of automobiles and housings for electrical/electronic appliances which people often touch or see over a long period of time. Furthermore, because of strict regulations protecting the environment, there is also an increasing need for a low gloss resin which is prepared without a coating process and which is directly moldable.
A widely used method used for preparing low gloss ABS resin with heat resistance partially replaces components of the ABS resin with heat resistant copolymers, such as styrene monomers, imide monomers and the like (John Scheirs Ed., “Modern Styrenic Polymers: Polystyrenes and Styrenic Copolymer, John Wiley & Sons Ltd, 2003, page 321˜340). A quencher can also be added to increase heat resistance of ABS resins. Examples of such heat resistant resins include α-methylstyrene-styrene-acrylonitrile (AMS-SAN) copolymer and N-phenyl maleimide-styrene-acrylonitrile (PMI-SAN) copolymer. In addition, inorganic loading materials, acrylic resins or cross-linked styrene resins can be added to lower gloss. Mixtures of such materials with other raw materials of the ABS resin can be melt-extruded.
In another method, a heat resistant ABS resin can be prepared using one of the above heat resistant copolymers, and gloss can be removed in a post processing step, for example, ejection molding using texture mold or a painting process. As yet another method, the roughness of the surface of a heat resistant ABS resin prepared using a heat resistant copolymer as described above can be controlled to have a micro-sized scale by controlling the size of the dispersed phase rubber particles. The surface prepared by above method reduces gloss by dispersing incident light.
One method for improving heat resistance and gloss of ABS resin includes melt extruding a heat resistant copolymer resin made using α-methylstyrene (AMS) or N-phenyl maleimide (PMI), a heat resistant copolymer resin matrix, and graft rubber with improved compatibility and additives. See U.S. Pat. No. 4,659,790, U.S. Pat. No. 4,757,109. Methods using additives to provide low gloss are convenient but quality can be irregular, depending on the dispersed phase. Further, the ratio of final products is increased by the high ratio of additives used.
Although extrusion processes using a texture mold or painting can be conducted without the additives, if a texture mold is used, processing costs can increase and pollution can also be a problem due to the additional processing step.
U.S. Pat. No. 5,475,053 discloses using a graft copolymer as a quencher and U.S. Pat. No. 4,652,614 discloses using a spherical graft copolymer which has about 5 to 80% of rubber to provide low gloss by controlling size and characteristics of the dispersed phase rubber particles. U.S. Pat. No. 4,668,737 discloses using spherical rubber particle having a core-shell structure with a size of about 0.05 to about 20 μm. Further, U.S. Pat. No. 5,237,004 discloses using polymer particles with a size of about 2 to 15 μm. However, using rubber particle additives can cause problems such as exfoliation, reduced physical properties or partially increased gloss, as well as result in high processing costs. Further it is difficult to provide a soft surface due to the fineness of such particles as compared with rubber particles prepared by bulk polymerization or solution polymerization.
A continuous bulk polymerization process can be used to control gloss and rubber particle diameter to provide a resin with both low gloss and heat resistance without requiring an extra process step. U.S. Pat. Nos. 5,091,470, 5,412,036, and 5,446,103 disclose a method for preparing heat resistant ABS resin using rubber including PMI monomer and butadiene by continuous polymerization. Heat resistant ABS resin prepared using PMI monomer can have both excellent heat resistance and high impact strength using a single process. However, in the above method, there is a difference between the composition of a graft polymer rubber prepared during an initial continuous polymerization step and a matrix composition prepared during a latter part of the continuous polymerization process due to the difference in the conversion ratio of the PMI monomer and styrene monomer during copolymerization. This can result in a loss of physical properties and impact strength. To overcome this problem, the composition can be adjusted by adding PMI according to degree of polymerization in each reactor, but this method is complicated.
It can be easy from a processing perspective to use liquid AMS monomer to prepare heat resistant ABS in a polymerization process and further the price of the raw materials is low compared to powder forms of PMI monomer. However, it is difficult to prepare a product having both excellent high heat resistance and high impact strength in a continuous ABS polymerizing process having a relatively high polymerization temperature because AMS monomer can self-depolymerize. Because the polymer melt is delivered in the continuous process at a temperature greater than the temperature at which PAMS is depolymerized, molecular weight can be reduced. Further, the rubber form can be destabilized during phase inversion polymerization of the rubber and matrix or polymerization after phase inversion. Accordingly, this method has limited usefulness for preparing products which have low heat resistance whether AMS monomer is used or not in a generally continuous ABS process.